A strain gauge typically comprises a conductor, such as a thin foil, arranged in a serpentine pattern. The pattern is such that the conductor extends along a length and doubles back on itself. This is repeated to form a plurality of parallel lengths.
The conductor is conventionally laid upon a flexible backing layer made from an insulative material. The backing layer isolates the conductor from a test component in which strain is to be measured. However, where the test component itself is an insulative material, the backing layer may be omitted. The strain gauge is affixed to the test component using an adhesive or other means of fixation.
The resistance of the conductor is measured in order to determine the strain of the test component. The resistance of the conductor varies depending on whether it is under compression or tension. When in compression, the length of the conductor decreases and its thickness increases, thus decreasing the resistance measured. Conversely, when in tension, the length of the conductor increases and its thickness decreases, which increases the resistance measured.
The change in length and thickness undergone during strain is experienced along each of the lengths of the conductor. Consequently, the change in resistance is multiplied by the number of lengths in the serpentine pattern. Therefore, the pattern amplifies the change in resistance measured, thus making the gauge far more sensitive.
Two strain gauges may be used to obtain a full surface 2D strain field. To achieve this, the two strain gauges are arranged perpendicular to each other. To improve the accuracy of this method, a third strain gauge may be included at an angle of 45 degrees to the other two strain gauges. This accounts for any misalignment between the strain gauges.
To improve the accuracy of the measurements, a “rosette” of strain gauges may be used to cancel out any voltage/current effects. Essentially, a strain gauge works as a variable resistor, and therefore they may be used to form a Wheatstone bridge. This provides a more sensitive and accurate measurement.
Generally, strain gauges are used for measuring strains that are slowly varying across the surface of the test component, and where the surface stresses are reasonably representative of the stresses that would be seen on the inside of the material of the component.
A component constructed from a composite material displays a different local strain adjacent to a fibre than in a region between fibres. It is conventional to use a larger strain gauge than usual, covering a large area of material, so as to average out such local strain variations. However, this is recognised to be a crude approach with limited value since it does not provide a detailed picture of the strain field across the surface.
It is an object of the present invention to seek to provide an improved strain gauge assembly.